An image forming apparatus, such as a copying machine, that uses an electrophotographic image formation method, adheres developer to an electrostatic latent image it forms on its image bearing member, such as a photosensitive drum, in order to develop the electrostatic latent image into a visible image. Some developing devices in accordance with the prior art have been known to use two-component developer made up of toner and magnetic carrier. These developing devices have been also known to use a method which develops an electrostatic latent image on their image bearing member (photosensitive drum), into a visible image, with the toner in the two-component developer, by conveying the developer to the immediate adjacencies of the image bearing member, with the use of their rotatable developer bearing member (which hereafter may be referred to simply as development sleeve) while keeping the two-component developer magnetically adhered to the developer bearing member.
Generally, these developing devices are provided with a development sleeve, a stationary magnet, and a developer regulating blade (which hereafter may be referred to simply as regulation blade). The stationary magnet is placed in the development sleeve to magnetically hold developer to the peripheral surface of the development sleeve. The regulation blade is positioned in the adjacencies of the development sleeve, with the presence of a preset amount of gap between itself and the peripheral surface of the development sleeve. Thus, the two-component developer is borne on the development sleeve, and is conveyed to the immediate adjacencies of the photosensitive member while being regulated in the amount to a preset value.
Conventionally, it has been a common practice to employ a development sleeve, the peripheral surface of which has microscopic peaks and valleys formed by blasting (sandblasting) with the use of microscopic particles, or multiple microscopic grooves which extend in parallel to the rotational axis of the development sleeve, in order to ensure that developer is reliably conveyed to the adjacencies of the peripheral surface of the photosensitive member.
However, a development sleeve, the peripheral surface of which has microscopic peaks and valleys formed by sandblasting is problematic in that if the microscopic peaks and valleys are smaller in dimension than a certain value, it is insufficient in performance in terms of developer conveyance. On the other hand, if a development sleeve needs to be increased in the dimension of the peaks and valleys of its peripheral surface, for the sake of increasing the development sleeve in developer conveyance performance, the process of sandblasting the peripheral surface of the development sleeve has to be increased in the intensity with which blasting particles are blasted upon the peripheral surface of the development sleeve, which is problematic in that the blasting process may deform the development sleeve. Generally, therefore, the sandblasted development sleeves which are currently in use are relatively small in the dimension of the peaks and valleys of their peripheral surface. However, in the case of a development sleeve which is small in the dimension of the peaks and valleys of its peripheral surface, its peaks and valleys are relatively quickly worn away by friction, compared to a development roller having relatively large peaks and valleys on its peripheral surface, while it is used for development for a substantial length of time, being therefore problematic in that it is not stable in the developer conveyance performance. This problem may become one of the reasons why a developing device is prematurely reduced in service life.
In recent years, an extremely high level of image quality, reliability, and stability have come to be required of a copying machine and a printer. From the standpoint of satisfying these requirements, it is very important to keep a development sleeve stable in the amount by which it conveys developer.
Thus, development sleeves having multiple grooves which extend in parallel to their axis have been proposed. One of such development sleeves is disclosed in Japanese Laid-open Patent Application H02-50182 (patent document 1). Unlike the method which uses sandblasting to provide the peripheral surface of a development sleeve with microscopic peaks and valleys, forming the abovementioned grooves in the peripheral surface of a development sleeve by putting a development sleeve through a die can provide the peripheral surface of the development sleeve with relatively large grooves (peaks and valleys), without causing the development sleeve to deform. Therefore, a development sleeve, the peripheral surface of which is provided with microscopic grooves with the use of a die is less likely to be affected by friction, being therefore more stable in developer conveyance performance, than a development sleeve, the peripheral surface of which has been sandblasted.
A development sleeve, the peripheral surface of which is provided with grooves is stable in terms of developer conveyance performance, but is problematic in that it requires the gap between itself and the aforementioned developer regulation blade to be relatively small, for the following reason. That is, providing the peripheral surface of a development roller with grooves can make the development sleeve stable in developer conveyance performance, but, it may make the development sleeve excessive in developer conveyance performance. Thus, it may require the gap between the development sleeve and regulation blade to be made relatively small to compensate for the excessive amount by which developer is conveyed by the development sleeve, because unless the gap is reduced, the development roller becomes excessive in the amount of the developer thereon.
Further, in recent years, an extremely high level of image quality has come to be required of an image forming apparatus. Therefore, in order to prevent, as much as possible, an image forming apparatus from becoming worse in the graininess attributable to the friction between the developer on a development sleeve and the toner image formed on the peripheral surface of a photosensitive member, there is a trend to reduce a developing device in the amount by which developer is borne by the peripheral surface of its development sleeve. Concretely, from the standpoint of keeping an image forming apparatus excellent in terms of the level of graininess in which it forms an image, the amount of developer per unit area of the peripheral surface of a development sleeve, on the downstream side of a developer regulation blade in terms of the rotational direction of the development sleeve, is desired to be set to a value in a range of (0.3±0.2) mg/mm2 (=(30±20) mg/cm2). More accurately, it is preferred that the amount by which developer is left coated on the peripheral surface of a development sleeve on the downstream side of the regulation blade is set in terms of the standardized specific gravity G (apparent thickness of developer coat). That is, the apparent thickness M/S of the developer coat on the peripheral surface of a development sleeve, in terms of specific gravity G, is desired to be in a range of 0.029-0.14 mm ((30±20) mg/cm2/3.48 mg/mm3)(M/S [mg/mm2]/specific gravity (density) G [mg/mm3])=0.029-0.14 mm ((30±20) mg/cm2/3.48 mg/mm3).
While a developing apparatus (device) is required to be less in the thickness of the developer on its development sleeve, there is a tendency that a developing device is further reduced in the gap between its development sleeve and regulation blade.
If the gap between a development sleeve and a regulation blade is rendered smaller than a certain value, such a problem is likely to occur that foreign substances, and the like, hang up in the adjacencies of the regulation blade and interfere with the developer coat on the development sleeve. Therefore, the gap between a development sleeve and a regulation blade is desired to be no less than 0.2 mm, preferably, no less than 0.3 mm.
On the other hand, reducing a developing sleeve in developer conveyance performance by imprudently reducing its grooves in depth, in order to allow the gap between the development sleeve and regulation blade to be widened, possibly makes the developer coat unstable, or causes the development sleeve to fail to be coated with the developer. Therefore, it is not desirable.